1. Field of the Invention
This invention relates to the Vacuum Arc Vapor Deposition of metallic and non-metallic thin films that bond permanently at the atomic level to the surface of a substrate and, in particular, to the deposition of multiple layers of materials to form a type of solid state circuit or microchip, e.g., an individual custom responsive circuit (sensor or transponder), integrated circuit or memory device.
2. Description of the Related Art
Microchips and responsive circuits, such as RFID tags, are routinely used for identification of objects and processing and storage of information related to such objects or their use. In most cases, the microchip is either bonded to a substrate with adhesive or it is embedded within a tag, e.g., between the layers of a plastic card. Sometimes, it is encased in a plastic cover to protect it.
Microchips for storage and transfer of information are available in many types. The passive types usually respond to proximity sensing or even touch and the active types typically are externally powered, such as with a battery. The active types typically transmit their response a greater distance and provide a much more flexible identification system, but, with such types, the size of the batteries prevent miniaturization of the entire circuit.
One limiting factor common to all microchips and tags is the attachment of the microchip or tag to the identified object. Microchips and tags that are bonded to objects with adhesives protrude from the surface of the objects and can either fall off or become damaged. They do not work well on moving parts or within an assembly as they can fall off and damage valuable equipment. Thus, their applications are limited since they are not integral with the substrate.
While microchips appear small and thin to the human eye, even the thinnest of these chips (100 microns or 4 thousandths of an inch) is too thick for many applications. In laminated devices (cards), the edge of the embedded microchip results in a raised portion on the outer surface of the card that both gives away the presence of the microchip and makes undesirable wrinkles in the printing area. However, having a microchip embedded or attached to the surface of a part appears to be the only way to store as much onboard information about the part as is required in some cases. Thus, the microchip industry is searching for ways to produce much thinner chips and more durable ways to attach them.
One method that has been tried is to print layers directly on the object, primarily using conductive inks, to form the circuit. While this printing method works fairly well to form certain conductive layers, as may be used in a passive RFID circuit, for example, one significant drawback is that this method is not capable of providing layers of many of the materials that are desirable in producing more complex layered circuits or microchips. The printing process relies on the ability of the substrate (usually paper) to be nonconductive and absorb the ink medium that carries some conductive material. This precludes use on a conductive surface. It also precludes layering circuits without additional layers of paper for the insulator. The present invention also provides for incorporating the dielectrics, a capability missing in paper labels. Further, printed layers are not formed as part of the object and, therefore, they may wear or wash off over time or with abuse, thereby destroying the functional characteristics of the circuits. Additionally, printed layers may outgas in a vacuum or evaporate under high temperatures. The present invention is durable and reliable under these conditions as it relies on materials joined at the atomic level.
In the context of simple marking of parts for identification by humans or machines, the aerospace industry has been seeking new marking methods that are safe and that can withstand harsh environments. The National Aeronautics and Space Administration (NASA) investigated a number of methods to spray and bond particles consisting of atoms and ions of source material onto surfaces. These included plasma-activated chemical vapor deposition, laser chemical deposition, sputtering, cathode-spot arc coating, electron beam evaporation deposition, ion plating, arc evaporation and cathode arc plasma deposition. However, all of the known processes tend to have relatively slow deposition rates compared to non-vapor coating methods. Consequently, NASA developed a Vacuum Arc Vapor Deposition (VAVD) apparatus, as described in U.S. Pat. No. 5,380,415 (which is herein incorporated by reference), consisting of a vacuum chamber system for producing vapor deposits. It utilizes the arc formed in a gas flowing through a hollow tungsten electrode in a substantially vacuum environment. The VAVD process is capable of very low or very high deposition rates and produces no hazardous wastes or by-products.
Tests conducted using the VAVD apparatus produced high quality thin film coatings including small, high fidelity human and machine-readable part identification symbols in seconds. The apparatus was also capable of producing single and multiple layer circuits. However, this apparatus was impractical for many uses because the size and operation of the vacuum chamber limited both the size and volume of parts being marked and, of course, it required operation within a vacuum chamber which adds significant complication to any manufacturing process.
In October of 2000, U.S. pat. app. Ser. No. 09/703,029 was filed by the National Aeronautics and Space Administration and has now issued as U.S. Pat. No. 6,395,151 B1 (the '151 patent). The '029 application (which is herein incorporated by reference) was filed on an improved VAVD device capable of depositing thin film layers directly on and integral with only a portion of a substrate, in essence, a focal VAVD device. For flexibility and field applications, the apparatus may be hand held and brought to any object or substrate needing its service. For stability and repeatability, the apparatus may be mounted to a robot or at a fixed station. In general terms, the device comprises a housing forming a chamber. Enclosed within the chamber are an electrode and a charge and the chamber includes a vacuum port and a deformable nozzle. A mask is placed between the nozzle and a substrate that is to receive the deposition, the mask including a pattern therethrough that is representative of the desired shape of the deposited material. With the nozzle sealed against the mask, a vacuum is drawn in the chamber. Next, the charge is at least partially vaporized by the electrode allowing the vapor to deposit on the portion of the substrate exposed through the mask.
Some of the materials that can be deposited using the VAVD process include pure metals such as aluminum, chromium, gold, molybdenum, nickel, silver, stainless steel, titanium and tungsten. Commonly used alloys include stainless steel, nickel-chromium, lead, tin and M—Cr—Al—Y. Typical compounds used in the process include Al203, TiC and TiB2. Silicon and many silicon compounds can also be deposited.
The invention of the '029 application overcomes many of the drawbacks and disadvantages of known marking methods and other thin film deposition devices. One capability of the disclosed device is to provide a means to clean and prepare a surface prior to applying a thin film deposition. The cleaning device preferably utilizes a high frequency generator and a cathode ring in close proximity to the part surface. This cleaning method removes contaminants and oxides from the area that will contain the deposited film layer.